Hierarchial control for discrete chiller units

ABSTRACT

System and process are disclosed for the hierarchial control of a plurality of discrete chiller units each having individual dedicated controls. The system and process imposes limits on the amount of compression that can be demanded by the individual dedicated controls. These limits are selectively changed in response to whether a level of chilling has been collectively achieved. The selective changing of these limits is premised on examinations of each individual chiller unit including the recent history of any changes in compression by the unit.

BACKGROUND OF THE INVENTION

This invention relates to the simultaneous control of several individualchiller units within a system that is processing chilled coolant viacommon supply and return lines. In particular, this invention relates tothe control of a number of different chiller units each having their ownrespective dedicated control units.

Multiple chillers on a common coolant loop are subjected to the sameentering coolant temperature at approximately the same time. Thechillers are typically required to maintain the leaving coolant at thesame temperature. This causes the individual units to simultaneouslystart or stop their compressors at the same time. This leads tounnecessary and at times excessive demands on electrical powerconsumption and excessive compressor cycling in order to provide thechilling of the coolant perceived to be necessary. In this regard, eachindividual chiller unit having its own particular compressor stages andcontrol unit is trying to activate the perceived number of compressorstages necessary to achieve the desired chilling without regard to whatmay be happening elsewhere. This often produces an over reaction.

What is needed is a hierarchial control system which allows individuallycontrolled units to provide normal control functions while at the sametime supervising reaction by the individual chiller units to maintainthe leaving coolant temperature at same predefined level.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a hierarchial control thatsynchronizes the individual control units of each chiller in order tomaintain the desired cooling.

It is another object of the invention to minimize electrical demand andeliminate unnecessary compressor cycling.

The above and other objects are achieved according to the presentinvention by providing a system which imposes demand limits onindividual chiller control units whereby the individual units cannotsimply activate or deactivate compressor stages. In the event that thecapacity demands placed on the individual units need to be exceeded, theoverall control system selects which individual units are to receivechanges in demand limits so as to thereby increase or decrease thenumber of compressor stages therein.

The decision as to when an individual chiller unit is to receive achange in demand limit is dependent on the values of certain systemlevel parameters. These parameters include the error between thetemperature of the coolant following chilling and a desired temperatureas well as the rate of change and duration in time of this temperaturedifferential. When a change in demand limit is authorized, a timer willbe set to measure whether a change in compression capacity occurs withina predefined time. The selected chiller unit in which the change is tooccur is characterized as ineligible in the event that the capacitychange does not occur within the predefined time. The selected chillerunit is otherwise characterized as having been the last one to add ordrop a stage of compression depending on the change that had beencommanded. These characterizations are thereafter used when the nextchiller unit to receive a change in demand limit is selected.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be apparentfrom the following description in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates a plurality of individually controlled chiller unitseach having a plurality of compressor stages located therein that may beactivated by a supervisory control system which interfaces with theindividually controlled chiller units;

FIG. 2 is a diagram of an overall software control process residing inthe supervisory control system of FIG. 1;

FIG. 3 is a diagram of a software routine which reads and storesinformation for each chiller unit of FIG. 1 as well as defining initialconditions of the entire system;

FIG. 4 is a diagram of a software routine which calculates the number ofchillers currently running and reads their operating capacityparameters;

FIG. 5 is a diagram of a software routine which transmits demand limitsand temperature set points to the chiller units of FIG. 1;

FIG. 6 is a diagram of a software routine which calculates certainsystem parameters for the overall system configuration of chiller unitsof FIG. 1;

FIG. 7 is a diagram of a software routine which checks to see whether achange has occurred in a chiller unit pursuant to an ordered change bythe supervisory control system of FIG. 1;

FIGS. 8A and 8B are a diagram of a software routine which selects thenext chiller units in which a stage of compression is to be added ordeleted;

FIG. 9 is a diagram of a software routine that calculates and otherwisedefines a parameter used to authorize a change in the demand limit ofone or more chiller units; and

FIG. 10 is a diagram of a subroutine which calculates a change in thedemand limit of a selected chiller unit in response to the softwareroutine of FIG. 9.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, a series of chiller units 10, 12 and 14 areillustrated relative to a coolant return line 16 and a coolant supplyline 18. The coolant will hereinafter be referred to as chilled wateralthough it should be understood that the system would work equally wellwith other coolants. Each of the chiller units includes individualcompressor stages such as 20, 22, and 24 for chiller unit 10 that can beindividually activated by a unit controller such as 26 for chiller unit10. In a similar manner, compressor stages can be activated by a unitcontroller 28 for chiller unit 12 and a unit controller 30 for thechiller unit 14. Each unit controller monitors the temperature of thechilled water in a line 18a, 18b, or 18c in order to exerciseappropriate local control over the respective chiller. This localcontrol may be in accordance with any number of well known controllermethods for activating or deactivating compressors in order to achievethe desired water supply temperature. In addition to controlling therespective chiller unit, each unit controller communicates with a systemcontroller 32 via a communication bus 34. The system controller 32receives local unit controller information from each controller unitover the bus 34 such as chiller status, unit capacity and availablecapacity. The system controller also receives certain information from auser interface 36 relating to the overall system of chillers. Thisinformation includes the number of chiller units in the system, thecoolant or chilled water set point, the overall system demand limit andthe minimum leaving chilled water control point temperature that are toapply to each chiller control unit. The system controller also receivesinformation about each controller unit from the user interface 36including the particular address for each unit controller, the effectivecapacity and the number of stages of compression control. The systemcontroller finally receives a chilled water supply temperature readingfrom a chilled water supply sensor 38 and a chilled water returntemperature reading from a chilled water return sensor 40.

Referring to FIG. 2, the programmed control within control system 32that is responsive to the aforementioned inputs is generallyillustrated. It is to be understood that this programmed control residesin a computer having sufficient memory and processing rate to executethe programmed control. The program control begins with a subroutine 44which reads the data entered at the user interface 36 and alsoinitializes the value of certain variables that will be used later. Thenormal program flow is to proceed to a subroutine 46 and extract theoperating data from the unit controllers 26, 28, and 30 and calculatethe number of chillers actually running. An initial demand limit andleaving chilled water control point are thereafter transmitted to eachof the control units. The initial demand limit which each control unitis given may not allow the activation of a sufficient number ofcompressor stages to meet the chilled water control point. The chillerswill nonetheless attempt to maintain their respective leaving chilledwater temperatures at the control point while not exceeding theirrespective demand limits. A step 50 simply determines whether there ismore than one chiller unit operating. In the event that there is not,the control program loops around to the subroutine 46. This allows theindividual operating chiller to operate under the control of its unitcontroller in maintaining the desired chilled water temperature.

Referring again to step 50, in the event that more than one chiller unitis active, the program proceeds in a subroutine 52 to read the chilledwater supply temperature from sensor 38 and the chilled water returntemperature from sensor 40 and calculate certain system parameters. Theprogram thereafter in a step 54 determines whether the chillers haveresponded to any previous commanded change in compression capacity. Ifchange is still pending, the program so notes in step 56 and returns tosubroutine 46.

In the event that a system change is not pending, the program proceedsto a subroutine 58 which selects the chillers that are to "add" or"drop" compression stages. This is determined by examining a number ofconditions for each chiller unit as will be described. hereinafterregarding subroutine 58. Knowing which chillers are to be changed, theprogram proceeds to calculate a variable CAPRATIO in a subroutine 60.The program will thereafter proceed to a subroutine 62 and change aselected chiller's demand limit if a change is in order as defined bythe determined value of CAPRATIO. Once this occurs, the program willloop around and begin again to execute the subroutines 46-62. It is tobe understood that this process will repeat itself every fifteenseconds.

Referring to FIG. 3, the steps comprising the system configurationroutine 44 of FIG. 2 are illustrated. This routine begins with a step 66wherein the number of chillers, N₋₋ CHIL, the system chilled water setpoint, CHWSPT, the system demand limit, DLM, and the minimum leavingchilled water controlled point, MINLCHW, are read from the userinterface 36. Since there are three chillers in FIG. 1, N₋₋ CHIL willequal three. It is to be understood that the number of chillers in thesystem may however be significantly greater. The system chilled waterset point, CHWSPT will be set at the temperature that is to be achievedby the coolant after chilling by the chillers 10-14. This temperaturewill ultimately be sensed by the chilled water supply sensor 38.

The system demand limit, DLM, is the maximum percentage capacity of eachchiller unit's compression capacity that may be activated by the unitcontroller for the system. The minimum leaving chilled water controlpoint, MINLCHW, will be set at some minimum temperature in degreesFahrenheit that is to be permitted for the coolant leaving each chillerunit.

The next step 68 is to read certain values for each of the individualchiller control units 26 through 30 from the user interface 36. Thisincludes reading the chiller address for the individual control unit,CHLRADDR, the effective chiller capacity, CHLCAP, and the number ofcompressor stages, NUM₋₋ STEP, for each particular unit controller.

The next step 70 initializes several variables that will be utilizedwithin the remainder of the program of FIG. 2. These variables includechilled water supply temperature, CHWSTEMP, which is set equal to thesystem chilled water set point, CHWSPT. The following variables are setequal to zero: ERROR, ERR₋₋ SUM, CAPRATIO, NEXTDROP, NEXTADD, UNITCAP₀and NEXTCHLR.

Referring now to FIG. 4, the routine for calculating the number ofchillers currently running is set forth. This routine begins by settinga variable N₋₋ RUN equal to zero in a step 72 and entering a loopbeginning with step 74 wherein each chiller from one to N₋₋ CHIL is readin a step 76 for chiller status, CHL₋₋ ST_(CHLR), the percent unitcapacity of compression for the given chiller unit, UNITCAP_(CHLR), andthe percentage of available compressor capacity that may be used,expressed as AVAILCAP_(CHLR). In the event that these values cannot besuccessfully read for a given chiller unit, then an attempt will be madein steps 78 and 80 to read each of these values two more times. In theevent that a given. chiller unit controller cannot be read the thirdsuccessive time then the chiller status, CHL₋₋ ST_(CHLR) for thatchiller is set equal to COMFAIL and the percent unit capacity,UNITCAP_(CHLR), and available capacity, AVAILCAP_(HLR) are set equal tozero in a step 82. For each chilled status, CHL₋₋ ST_(CHLR), that issuccessfully read, and is equal to "ON" or "RESTART" the variable N₋₋RUN is incremented in step 86 by one. This will result in N₋₋ RUNfinally storing a count of chillers that are running by the end of theroutine defined by step 88.

Referring to FIG. 5, the routine for transmitting the demand limit andthe leaving chilled water control point for each chiller is illustratedin detail. It will be remembered that this is routine 48 in FIG. 2. Thisroutine begins with a step 90 which defines a subscript variable, CHLR,that is to be incremented from one to N CHIL. This subscript variableidentifies certain parameters of a given chiller that are to be dealtwith in a manner which will now be described. If N₋₋ RUN indicates thatone or fewer chillers are actually running in step 92, then the activedemand limit, CHL₋₋ DLM_(CHLR) for each chiller is set equal to thesystem demand limit DLM and the leaving chilled water control pointLCWCTLPT_(CHLR) for each chiller is set equal to chilled water setpoint, CHWSPT in step 94. These values will subsequently be written tothe respective control unit for the identified chiller in step 96. Inthe event that more than one chiller is running, a different path willbe defined out of step 92 to a step 98. The leaving chilled watercontrol point, LCWCTLPT_(CHLR), for each chiller will be set equal tothe minimum leaving chilled water control point, MINLCHW, in a step 98.This minimum leaving chilled water control point will be substantiallydifferent than the chilled water set point, CHWSPT. In this regard, theminimum leaving chiller water control point, MINLCHW, will be anartificially low value that cannot in fact be achieved by the chillersoperating under the demand limit, CHL₋₋ DLM_(CHLR), that is initiallyplaced on each of them. This combination of the low value of MINLCHW andthe demand limit, CHL₋₋ DLM_(CHLR), for each chiller allows the systemcontroller 32 to supervise the further adding or dropping of compressorsin the chiller units in a manner which will be described in detailhereinafter.

The routine now proceeds to examine certain control parameters in steps100, 102, and 104. These control parameters are CAPRATIO, NEXTDROP andthe subscript variable, CHLR. It is to be appreciated that thevariables, CAPRATIO and NEXTDROP will initially have the value of zeroassigned to them in step 70. The subscript variable, CHLR, will on theother hand take an successive values of one, two and three for the threechillers 10, 12 and 14 of FIG. 1. This will cause the routine toimmediately drop through to a step 106 for each numbered chillerallowing the demand limit for each chiller to be initially set equal tothe percent unit capacity for that chiller. It will be remembered thatthis percent unit capacity is the percentage compression capacity of achiller unit that is currently in use. The particular demand limit thusdefined in step 106 and the leaving chilled water control point definedin step 98 will be written to the identified chiller control unit instep 96. The routine will loop around through step 108 and increment thesubscript variable, CHLR until all chiller control units have had theirdemand limits and leaving chilled water control points defined in step96.

Referring to FIG. 6, the routine 52 for calculating system parameters isillustrated in detail. The value of chilled water supply temperature,CHWSTEMP, is saved as PRV₋₋ CHWS in a step 110. The chilled water supplytemperature, CHWS, is next read from the temperature sensor 38 and savedas the new value of CHWSTEMP in steps 112 and 114. The average chilledwater supply temperature, AVG₋₋ CHWS, is next calculated in step 116 asbeing the average of the currently read chilled water supply temperaturein step 114 and the previous chilled water supply temperature of step110. The routine next proceeds to a step 118 and saves the current valueof ERROR in the variable PRVERROR. The routine then calculates an errorbetween the average chilled water supply temperature AVG₋₋ CHWS and thechilled water set point, CHWSPT. It will be remembered that CHWSPT wasread in step 66 as one of the system parameters provided by the userinterface. The routine now calculates an error rate ERR₋₋ RATE in a step122 which is the differential between the currently calculated error ofstep 120 and the previous error of step 118 multiplied by four. Thesignificance of the number four is that the entire program of FIG. 2 isrun once every fifteen seconds so that a multiplication by four of thedifferential between current and previously calculated error effectivelygives an error rate that is expressed per minute of unit time.

The routine will next determine the current capacity being used in thesystem by all chillers in a step 124. This is accomplished bymultiplying the effective capacity, CHL CAP, times the percent unitcapacity, UNITCAP, divided by one hundred for each chiller that has beennoted in the sub routine 46 as having a status of either being "on orrestart". The results for each chiller are thereafter summed in step 124to be the value of CUR₋₋ CAP.

The routine of FIG. 6 now proceeds to a step 126 and reads the chilledwater return temperature, CHWRT from sensor 40. The variable, RISE₋₋ KW,is next calculated in a step 128. Referring to step 128, the differencebetween the chilled water return temperature, CHWRT, read from sensor 40in step 126 and the average chilled water supply temperature, AVG₋₋CHWS, calculated in step 116 is divided by the calculated currentcapacity being used in the system, CUR₋₋ CAP, determined in step 124.The calculated result is expressed in degrees of Fahrenheit temperatureper kilowatt unit of thermal energy. This result is used as anindication of how much cooling is being achieved per unit of chillingcapacity being operated by the system.

The routine now proceeds to a step 130 and calculates a value for thevariable, ERR₋₋ SUM, which is a summation of three times the error rate,ERR₋₋ RATE, calculated in step 122 plus the error, ERROR, calculated instep 120 divided by two plus the previous error sum, ERR₋₋ SUM. It is tobe appreciated that this calculation will initially be an averaging ofthe error plus three times the error rate since error sum is initiallyset equal to zero in step 70. This step will ultimately be anintegration or summation as the routine continues to execute relative toa previous calculated error sum.

Referring now to FIG. 7, the routine 54 of FIG. 2 which calculateswhether a change is pending is illustrated in detail. This routinebegins with a series of steps 134 and 136 which inquire as to whetherCAPRATIO is greater than or equal to one (step 134) or less than orequal to minus one (step 136). If CAPRATIO is anything between theselimits the routine proceeds to a step 138 and sets a variableOLDCAP_(NEXTCHLR) equal to the value of the percent unit capacity of thechiller identified by the variable UNITCAP_(NEXTCHLR). It will beremembered that UNITCAP_(CHLR) is read in for each chiller in step 76and is therefore available for a given chiller. The particular percentunit capacity that is however initially chosen in step 138 is the onethat is identified by the initial value of the subscript NEXTCHLR. Sincethe value of NEXTCHLR is initially set equal to zero, this will meanthat percent unit capacity for an imaginary chiller number zero will beinitially selected. The percent unit capacity for this imaginary chilleris UNITCAP₀, which is defined in step 70 as being zero.

Referring again to step 134, if CAPRATIO is greater than or equal toone, then an increase in capacity is pending. As will be explained indetail hereinafter. The routine proceeds to a step 140 and inquires asto whether the percent unit capacity of the next chiller that has beenselected to add a stage of compression has in fact done so. This isaccomplished by inquiring as to whether the value of the percent unitcapacity, UNITCAP_(NEXTCHLR) is greater than that chiller's old percentUnit capacity, OLDCAP_(NEXTCHLR), plus two percent. The two percentvalue is arbitrarily chosen to cover any possible sizes of capacity thatmay be added from a possible two percent to fifty percent.

If the percent unit capacity, UNITCAP_(NEXTCHLR), has in fact increasedby two percent, then the identified chiller value stored in thevariable, NEXTCHLR, is stored in the variable LASTADD in a step 142.Referring to the next step 144, the following is seen to occur: ERR₋₋SUM previously calculated in step 130 is set equal to zero as is achange timer variable, CHGTIMER. The latter variable is a software clockwhich will have been steadily increasing in time as will become apparenthereinafter. The chiller demand limit for the chiller identified byNEXTCHLR will be set equal to the value of its percent unit capacity,UNITCAP_(NEXTCHLR). Finally a change pending variable, CHNGPNDG, will beset equal to "false". The routine will proceed to step 138 and set thefinally achieved percent unit capacity of the identified chiller equalto the "old capacity" for that chiller.

Referring again to step 140, if a pending change in UNITCAP_(NEXTCHLR)has not yet occurred, then the change timer clock variable, CHGTIMER, ischecked in a step 146. Changes in UNITCAP_(NEXTCHLR) are allowed tooccur for a period of up to one hundred twenty seconds. If the changetimer has not expired, the routine proceeds to step 147 and sets thevariable change pending, CHGPNDG, equal to "true". The routine willthereafter implement a delay of fifteen seconds in a step 148. Followingthe fifteen second delay, the routine will exit through step 138 whichperforms the old capacity calculation.

Referring to FIG. 2, upon exiting the routine 54, the change pendingstatus is queried. If the change pending variable, CHGPNDG equals "true"the program proceeds back to subroutine 46 and again executessubroutines 46 through 52 before again encountering the routine 54 ofFIG. 7. This will continue to occur until the change timer expires instep 146 or the unit capacity has increased in step 140. When timeexpires before unit capacity increases, the chiller is removed fromselection eligibility in a step 149 and the variable CHGPNDG is setequal to false in step 150 before the routine exits through step 138.Referring again to step 56 in FIG. 2, once the variable, CHGPNDG, isfalse, the program will no longer execute the change pending loop backto routine 46.

Referring again to step 134 in FIG. 7, if CAPRATIO is not greater thanor equal to one, the routine proceeds to step 136 and determines whetherCAPRATIO is less than or equal to minus one. In the event that thedetermination is "yes" than a decrease in capacity is pending in thechiller identified by the variable, NEXTCHLR, and the routine proceedsto steps 152, 154 and again 144 in much the same manner as has beenpreviously described with regard to the processing of a change in unitcapacity in steps 140 and 142. In the event that a decrease has yet tooccur, then the steps 146-150 are appropriately utilized with steps 56and 58 also being encountered depending on the status of the changepending variable, CHGPNDG as previously discussed.

Referring to FIG. 8A, the routine 58 of FIG. 2 is further illustrated.This routine computes both the next chiller to add a stage ofcompression as well as the next chiller to drop a stage. In this regard,the variables NEXTADD and NEXTDROP are initially set equal to zero in astep 160. The routine proceeds in a step 162 to identify the nextchiller that is to add a stage. This is accomplished by identifying thefirst chiller to have a status of "on" in a step 164 and to also nothave an available capacity of one hundred percent as determined in astep 166. The chiller must also have not been the last to drop a stageof capacity and to also have added a stage capacity as indicated byLASTADD and LASTDROP in a step 168. The routine shall finally excludeany chiller that was declared ineligible in a step 170 if it failed toexecute its last commanded change. It will be remembered that a chillermay be ruled ineligible in step 149 if it failed to implement a changein capacity within the predefined period of time. The first chiller topass each of the inquiries 164 through 170 is identified in step 172 asthe next chiller to add a stage of compression. This is done by storingthe chiller's number, CHLR, in the variable NEXTADD.

Referring to FIG. 8B, step 174 defines a successive interrogation ofeach chiller unit's controller for the purpose of identifying the nextchiller that is to drop a stage of compression. Step 176 determineswhether the chiller is on, step 178 determines whether the chiller has apercent unit capacity greater than zero and whether the chiller was lastto drop a capacity followed by a request to add capacity as indicated instep 180. Finally the chiller is excluded if it has been previouslydeclared ineligible in step 149 because it failed to execute its lastcommand as indicated by step 182. The first chiller to successively passeach of these thresholds is identified in step 184 by setting thevariable NEXTDROP equal to the identified chiller number.

Referring now to FIG. 9, the routine 60 of FIG. 2 for calculatingCAPRATIO is illustrated in detail. This routine begins with a step 186which follows the end of the previous routine for selecting the nextchiller to add a stage of compression or the next chiller to drop astage of compression. Referring to step 186, the variable, ERROR, ischecked for being less than Zero. It will be remembered that the valueof this particular variable is calculated in a step 120 and reflects thedifference between the average chilled water supply temperature and thechilled water set point. If the value of ERROR is less than zero, thanthe next chiller that is to drop a stage as indicated by NEXTDROP isstored in the variable NEXTCHLR in step 188. If on the other hand, theerror is greater than or equal to zero, the routine stores theidentification of the next chiller to add a stage, NEXTADD in thevariable NEXTCHLR in a step 190. Whichever chiller is identified ineither step 188 or step 190, the routine proceeds to a step 192 andcalculates STAGSIZE for this specified chiller. This is essentially acalculation of the chilling capacity of a given stage of compressionthat is to be added or deleted for the identified chiller, NEXTCHLRdivided by the number of stages of compression for that chiller. Thecalculation is premised on knowing the chilling capacity and number ofcompression stages of the specified chiller as a result of having readand stored these-values for that chiller in step 68. It is to beunderstood that STAGSIZE will be a value of the chilling capacity perstage of compression of the selected chiller expressed in kilowatts perstage.

The routine proceeds to calculate the integration limit for theidentified chiller, INT₋₋ LIM, in a step 194. It is to be noted thatthis is calculated by multiplying the previously calculated value ofSTAGSIZE by a factor of four and further multiplying the result by thevalue of RISE₋₋ KW and adding ten to the resulting product. It will beremembered that the value of RISE₋₋ KW was previously calculated in step128 of the system parameter routine as being the difference between thechilled water return temperature and the average chilled water supplytemperature divided by the value of current capacity of the system. Itis to be appreciated that this calculated value is expressed in degreesof Fahrenheit temperature per unit of chilling capacity. Referring againto step 194, it is to be appreciated that a multiplication of the valueof RISE₋₋ KW times the value of STAGSIZE will yield degrees oftemperature in Fahrenheit per stage.

The routine proceeds to a step 196 and determines whether or not theaforementioned sequence of steps 186 through 194 has occurred for thefourth time. If not, the value of CAPRATIO is set equal to zero in astep 198. At the fourth time, the routine calculates the value ofCAPRATIO in step 200 as being the value of ERR₋₋ SUM divided by thevalue of INT₋₋ LIM. it will be remembered that ERR₋₋ SUM is calculatedin step 130 of the system parameter calculation routine. This isessentially anintegrated error calculation comprising the summation ofthe system's currently calculated error and error rate values plus theprevious value of ERR₋₋ SUM and is expressed as degrees of Fahrenheittemperature. As has been noted, above, INT₋₋ LIM is expressed as degreesof temperature in Fahrenheit per stage. This means that the variableCAPRATIO is a ratio of degrees of Fahrenheit temperature divided bydegrees of Fahrenheit per stage which yields "stages". This resultexpressed in terms of number of stages, is used to indicate that a stageof chilling should be added if it is plus one or dropped if it is minusone.

It is to be appreciated that at this point in time the value of CAPRATIOis either the result of the calculation of steps 198 or 200. The valueof CAPRATIO that has been arrived at is next used in the routine of FIG.10 to determine whether a change in the demand limit for an identifiedchiller is necessary. It will be remembered that this is a routine 62 inFIG. 2.

Referring to a step 202 in FIG. 10, if the value of CAPRATIO is greaterthan or equal to one, then the routine proceeds to a step 204 andcalculates a chiller demand limit, CHL₋₋ DLM_(NEXTADD), for the chillerthat has been previously identified as the chiller to next add a stageof compression. It will be remembered, that this chiller is identifiedby the variable NEXTADD in step 172 of the routine of FIG. 8. The demandlimit for this identified chiller is calculated as being equal to thepercent unit capacity, UNITCAP_(NEXTADD), for the identified chillerplus an increment thereto. The increment is defined by the number twohundred divided by the number of stages of compression, NUM₋₋ STEP, forthe identified chiller. The number two hundred is chosen so that theratio "200/NUM₋₋ STEP_(NEXTADD) " will be two times the likelypercentage of the average stage size. This allows for the addition ofthe largest potential chilling stage that can be added. Once the newdemand limit has been calculated, the routine proceeds to a step 206 andstarts a change timer clock denoted as CHGTIMER. The routine nowproceeds to a step 208 which marks the end of the routine of FIG. 10.Step 208 will initiate any delay that is necessary in order to define asufficient amount of time that needs to elapse before the routine ofFIG. 10 actually ends. This preferably is enough time to allow theprogram of FIG. 2 to execute every fifteen seconds. Referring to FIG. 2,it is to be appreciated that upon exiting routine 62 of FIG. 10 theprogram returns to routine 46 whereupon a new calculation of the numberof chillers running is made. The program will thereafter address thedemand limit and control point situation in the routine 48. Referring toFIG. 5, the subroutine 48 is illustrated in detail. Since CAPRATIO isgreater than one, step 104 will be triggered when the identified chillerto add a stage is encountered. The chiller demand limit for that chilleras calculated in step 204 will next be written to the control unit forthat chiller in step 96. At this point, the chiller control unit isauthorized to actually increase the compression capacity of the givenchiller unit. The program of FIG. 2 will thereafter check in subroutine54 to determine whether the change in the. unit capacity has in factoccurred. This is accomplished in step 140 of FIG. 7 wherein the percentunit capacity of the next chiller designated to add capacity is examinedfor whether such has occurred. Since the variable NEXTCHLR was set equalto NEXTADD in step 190, the chiller examined in step 140 will be thesame chiller whose demand limit was increased in step 96. In the eventthat the change occurs before the change timer has clocked out, theprogram proceeds to denote the chiller as being the "last to add" instep 142 and thereafter resets the parameters of ERR SUM, CHL₋₋DLM_(NEXTCHLR), CHG₋₋ TIMER and CHNGPNDG in step 144. The variable,OLD₋₋ CAP, denoting the old capacity of the chiller unit will also beupdated in step 138 before the system is again ready to select the nextchiller that will add a stage of compression in the routine 58. It is tobe appreciated that the program of FIG. 2 will continue increasingdemand limits of selected chillers until the value of CAPRATIO is deemedacceptable in the routine 62 of FIG. 10 so as to not require any furtherchanges in the demand limits for the chillers of the system.

Referring again to FIG. 10, and in particular to a step 210, if CAPRATIOis at anytime less than or equal to minus one, the routine proceeds to astep 212 and calculates the demand limit for the identified chiller thatis to drop a stage of compression. This demand limit is seen to be theunit capacity, UNITCAP_(NEXTDROP) minus the constant fifty divided bythe number of compressor stages for the identified chiller. The numberfifty is chosen so that the ratio of "50/NUM₋₋ STEP_(NEXTDRP) " is equalto one-half the percentage of an average stage size's percentage oftotal capacity. It is to be appreciated that this calculation isdefining the percentage of the smallest stage of compression so thatonly one stage will be dropped. The routine will proceed from step 212to start the change timer clock and thereafter implement the delay ofstep 208 before eventually writing the calculated demand limit into theidentified chiller in step 96 as a result of being triggered by step 102in the routine 48. The decrease in unit capacity expected in theidentified chiller will be checked for in step 152 of the routine 54.The change in unit capacity should occur within the time allotted by thechange timer that is of course set in step 206 and measured in step 146.As has been previously discussed, if the unit capacity changes withinthe allotted time, the chiller is denoted as the last to drop a stage instep 154 and the values of ERR₋₋ SUM, CHL₋₋ DLM, CHG₋₋ TIMER, andCHNGPNDG are again reset in step 144. The program of FIG. 2 willcontinue to select chillers and selectively decrease demand limits untilthe CAPRATIO is within acceptable limits as required by the routine 62of FIG. 10.

It is to be appreciated that a complete process has been disclosed foridentifying chiller units that are to be selected for the addition orsubtraction of stages of compression depending upon the history of thatchiller and the status of the system. The status of the system isreflected in the calculated capacity ratio, CAPRATIO. In each instancewhere values of CAPRATIO exceed predefined limits, demand limits areselectively changed so as to thereby control which chiller units may addor delete stages of compression within the system.

It is furthermore to be appreciated that a particular embodiment of theinvention has been described. Alterations, modifications, andimprovements thereto will readily occur to those skilled in the art.Such alterations, modifications and improvements are intended to be partof this disclosure even though not expressly stated herein and areintended to be within the scope of the invention. Accordingly theforegoing description is by way of example only. The invention islimited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. A process for selectively adding or droppingstages of compression within discrete chiller units each chiller unithaving a dedicated control unit associated therewith for normally addingor dropping stages of compression in the particular chiller unit, saidprocess comprising the steps of:defining a demand limit on thecompression capacity in each respective chiller unit that limits thetotal compressor capacity that may be activated by the dedicated controlunit for that respective chiller unit, each dedicated control unitthereafter independently activating stages of compression subject onlyto the defined demand limit; and changing the demand limit on thecompression capacity of at least one chiller unit when a desired coolanttemperature is not being achieved by the collective compression capacityof all discrete chiller units that have been activated by theirrespective control units whereby the dedicated control unit for thatrespective chiller unit may thereafter change the number of stages ofcompression in that respective chiller unit subject to the new demandlimit on the compression capacity for that respective chiller unit. 2.The process of claim 1 wherein said step of changing the demand limit onthe compression capacity of at least one chiller unit comprises:sensingthe temperature of the coolant following compression by the discretechiller units; defining an error between the sensed temperature of thecoolant and the desired coolant temperature; calculating an error rateat which the defined error between the sensed temperature of chilledcoolant following compression and a desired temperature is changing overtime; calculating an error sum value that is a function of defined errorand the error rate; calculating a temperature contribution value to bemade by a stage of compression within a chiller unit that has beenselected; calculating a ratio of the calculated error sum value dividedby the calculated temperature contribution value; increasing the demandlimit on the compression capacity of at least one chiller unit when thecalculated ratio exceeds a first predefined level; and decreasing thedemand limit on the compression capacity of at least one chiller unitwhen the calculated ratio is less than a second predefined level.
 3. Theprocess of claim 2 wherein said step of increasing the demand limit ofat least one chiller comprises:identifying a chiller unit that is to bethe next to add a stage of compression; obtaining the amount ofcompression capacity currently being used by the identified chillerunit; and adding a calculated amount of the compression capacity of theidentified chiller unit to the amount of compression capacity currentlybeing used by the identified chiller unit so as to define a newincreased demand limit for the identified chiller unit.
 4. The processof claim 3 wherein said step of calculating a new increased demand limitfor the identified chiller further comprises the step of:calculating anamount of compression capacity to be added to the identified chillerunit's compression capacity currently being utilized, the calculatedamount being sufficient to include the amount of compression capacityrepresented by the largest stage of compression for that chiller unit.5. The process of claim 2 wherein said step of decreasing the demandlimit of at least one chiller unit comprises the steps of:identifying achiller unit that is to be the next to drop a stage of compression:obtaining the amount of compression capacity currently being used by theidentified chiller unit; and subtracting a calculated amount of thecompression capacity of the identified chiller unit from amount ofcompression capacity currently being used by the identified chiller unitso as to define a new decreased demand limit for the identified chillerunit.
 6. The process of claim 5 wherein said step of decreasing thedemand limit of an identified chiller further comprises the stepof:calculating an amount of compression capacity to be subtracted fromthe identified chiller unit's compression capacity currently beingutilized, the calculated amount being sufficient to include the smalleststage of compression for that chiller unit.
 7. The process of claim 2wherein said step of calculating a temperature contribution value by astage of compression within a chiller unit that has been selectedcomprises the steps of:calculating a change in temperature per unit ofcurrent active compressor capacity in the system; calculating thecompressor capacity of the stage of compression within the chiller unitthat has been selected; and multiplying the calculated change intemperature per unit of current compressor capacity by the calculatedcompressor capacity.
 8. The process of claim 2 wherein said step ofcalculating an error sum value comprises the step of:summing theprevious error sum value with a fractional value of the defined errorplus a fractional value of the calculated error rate.
 9. The process ofclaim 1 further comprising the steps of:determining when an additionalstage of compression should be added to or dropped from the collectivecompression capacity of all discrete chiller units that have beenactivated by their respective dedicated control units; and triggeringsaid step of changing the demand limit on the compression capacity ofone chiller unit in response to said step of determining when anadditional stage of compression should be added to or dropped from thecollective compression capacity of the active discrete chiller units.10. The process of claim 9 wherein said step of changing the demandlimit on the compression capacity of one chiller unit comprises thesteps of:identifying the chiller unit that is to receive a change indemand limit; calculating a new demand limit for the identified chillerunit, and transmitting the new demand limit to the dedicated controlunit for the identified chiller unit.
 11. The process of claim 10wherein said step of identifying the chiller unit that is to receive achange in demand limit comprises the steps of:designating the nextchiller unit that is to add a stage of compression; designating the nextchiller unit that is to drop a stage of compression; identifying thedesignated next chiller that is to add a stage of compression as thechiller that is to receive a change in demand limit when the temperatureof the coolant leaving the chiller units is greater than the desiredcoolant temperature; and identifying the designated next chiller that isto drop a stage of compression as the chiller unit that is to receive achange in demand limit when the temperature of the coolant leaving thechiller units is less than the desired coolant temperature.
 12. Theprocess of claim 10 wherein said step of calculating a new demand limitfor the identified chiller unit comprises the steps of:obtaining theamount of compression capacity currently being used by the identifiedchiller unit; adding a calculated amount of compression capacity of theidentified chiller unit to the amount of compression capacity currentlybeing used by the chiller unit when a stage of compression is to beadded; and subtracting a calculated amount of compression capacity ofthe identified chiller unit from the amount of compression capacitycurrently being used by the identified chiller unit when a stage ofcompression is to be dropped.
 13. The process of claim 12 wherein thecalculated amount of compression capacity to be added to the amount ofcompression capacity currently being used is sufficient to include theamount of compression capacity represented by the largest stage ofcompression of the identified chiller unit and wherein the calculatedamount, of compression capacity to be subtracted from the amount ofcompression capacity currently being used is sufficient to include thesmallest stage of compression of the identified chiller unit.
 14. Theprocess of claim 1 further comprising the steps of:checking whether achange in active compression capacity has occurred within a predefinedperiod of time in a chiller unit that has had its demand limit forcompression capacity changed.
 15. The process of claim 14 furthercomprising the steps of:designating the chiller unit that has had itsdemand limit for compression capacity changed as being the last to addin the event the demand limit for compression capacity increased or thelast to drop in the event the demand limit for compression capacitydecreased within the predefined period of time; and selecting the nextchiller that is to receive a change in demand limit, said selectionexcluding a chiller unit from being selected that has been designated asbeing the last to add and the last to drop.
 16. The process of claim 14further comprising the steps of:designating a chiller unit as ineligibleto receive a further change in demand limit in the event the chillerunit being checked for change has not changed its active compressioncapacity by a predefined amount during the predefined period of time;and selecting the next chiller that is to receive a change in demandlimit, said selecting means excluding a chiller unit from being selectedthat has been designated by said designating means as being ineligible.17. The process of claims 15 or 16 wherein said steps of selecting thenext chiller unit that is to receive a change in demand limitincludes:excluding a chiller unit that does not have availablecompression capacity to respond to the change in demand limit.